1.9 The `Py_BuildValue()` Function

This function is the counterpart to PyArg_ParseTuple(). It is declared as follows:

PyObject *Py_BuildValue(char *format, ...);

It recognizes a set of format units similar to the ones recognized by PyArg_ParseTuple(), but the arguments (which are input to the function, not output) must not be pointers, just values. It returns a new Python object, suitable for returning from a C function called from Python.

One difference with PyArg_ParseTuple(): while the latter requires its first argument to be a tuple (since Python argument lists are always represented as tuples internally), Py_BuildValue() does not always build a tuple. It builds a tuple only if its format string contains two or more format units. If the format string is empty, it returns None; if it contains exactly one format unit, it returns whatever object is described by that format unit. To force it to return a tuple of size 0 or one, parenthesize the format string.

When memory buffers are passed as parameters to supply data to build objects, as for the "s" and "s#" formats, the required data is copied. Buffers provided by the caller are never referenced by the objects created by Py_BuildValue(). In other words, if your code invokes malloc() and passes the allocated memory to Py_BuildValue(), your code is responsible for calling free() for that memory once Py_BuildValue() returns.

In the following description, the quoted form is the format unit; the entry in (round) parentheses is the Python object type that the format unit will return; and the entry in [square] brackets is the type of the C value(s) to be passed.

The characters space, tab, colon and comma are ignored in format strings (but not within format units such as "s#"). This can be used to make long format strings a tad more readable.

"s" (string) [char *]: Convert a null-terminated C string to a Python object. If the C string pointer is NULL, None is used.
"s#" (string) [char *, int]: Convert a C string and its length to a Python object. If the C string pointer is NULL, the length is ignored and None is returned.
"z" (string or None) [char *]: Same as "s".
"z#" (string or None) [char *, int]: Same as "s#".
"u" (Unicode string) [Py_UNICODE *]: Convert a null-terminated buffer of Unicode (UCS-2) data to a Python Unicode object. If the Unicode buffer pointer is NULL, None is returned.
"u#" (Unicode string) [Py_UNICODE *, int]: Convert a Unicode (UCS-2) data buffer and its length to a Python Unicode object. If the Unicode buffer pointer is NULL, the length is ignored and None is returned.
"u" (Unicode string) [Py_UNICODE *]: Convert a null-terminated buffer of Unicode (UCS-2) data to a Python Unicode object. If the Unicode buffer pointer is NULL, None is returned.
"u#" (Unicode string) [Py_UNICODE *, int]: Convert a Unicode (UCS-2) data buffer and its length to a Python Unicode object. If the Unicode buffer pointer is NULL, the length is ignored and None is returned.
"i" (integer) [int]: Convert a plain C int to a Python integer object.
"b" (integer) [char]: Same as "i".
"h" (integer) [short int]: Same as "i".
"l" (integer) [long int]: Convert a C long int to a Python integer object.
"c" (string of length 1) [char]: Convert a C int representing a character to a Python string of length 1.
"d" (float) [double]: Convert a C double to a Python floating point number.
"f" (float) [float]: Same as "d".
"O" (object) [PyObject *]: Pass a Python object untouched (except for its reference count, which is incremented by one). If the object passed in is a NULL pointer, it is assumed that this was caused because the call producing the argument found an error and set an exception. Therefore, Py_BuildValue() will return NULL but won't raise an exception. If no exception has been raised yet, PyExc_SystemError is set.
"S" (object) [PyObject *]: Same as "O".
"U" (object) [PyObject *]: Same as "O".
"N" (object) [PyObject *]: Same as "O", except it doesn't increment the reference count on the object. Useful when the object is created by a call to an object constructor in the argument list.
"O&" (object) [converter, anything]: Convert anything to a Python object through a converter function. The function is called with anything (which should be compatible with void *) as its argument and should return a ``new'' Python object, or NULL if an error occurred.
"(items)" (tuple) [matching-items]: Convert a sequence of C values to a Python tuple with the same number of items.
"[items]" (list) [matching-items]: Convert a sequence of C values to a Python list with the same number of items.
"{items}" (dictionary) [matching-items]: Convert a sequence of C values to a Python dictionary. Each pair of consecutive C values adds one item to the dictionary, serving as key and value, respectively.

If there is an error in the format string, the PyExc_SystemError exception is raised and NULL returned.

Examples (to the left the call, to the right the resulting Python value):

    Py_BuildValue("")                        None
    Py_BuildValue("i", 123)                  123
    Py_BuildValue("iii", 123, 456, 789)      (123, 456, 789)
    Py_BuildValue("s", "hello")              'hello'
    Py_BuildValue("ss", "hello", "world")    ('hello', 'world')
    Py_BuildValue("s#", "hello", 4)          'hell'
    Py_BuildValue("()")                      ()
    Py_BuildValue("(i)", 123)                (123,)
    Py_BuildValue("(ii)", 123, 456)          (123, 456)
    Py_BuildValue("(i,i)", 123, 456)         (123, 456)
    Py_BuildValue("[i,i]", 123, 456)         [123, 456]
    Py_BuildValue("{s:i,s:i}",
                  "abc", 123, "def", 456)    {'abc': 123, 'def': 456}
    Py_BuildValue("((ii)(ii)) (ii)",
                  1, 2, 3, 4, 5, 6)          (((1, 2), (3, 4)), (5, 6))

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1.9 The Py_BuildValue() Function

1.9 The `Py_BuildValue()` Function